Various disclosures exist that describe several types of techniques that have been employed for the purification of organometallic compounds, such as trimethylaluminum, including the following:
Japanese Patent Publication No. 08/12,678 describes the use of a cooling tube to precipitate, from a melt composition of an organometallic compound, the desired organometallic compound in purified form. This technique treats a neat composition of the organometallic compound that is to be purified.
Apparatus that are especially designed for directional crystallization are described as being suitable for the purification of an organometallic compound in Chemical Abstracts, Vol. 106, 84687 (1999).
U.S. Pat. No. 4,362,560 describes the use of a vacuum-thermal decomposition procedure to produce high purity gallium from gallium-arsenic containing waste.
Adduct formation between the organometallic compound and an adduct-forming reagent followed by later dissociation by thermal means is described in PCT International Patent Publication Nos. WO 85/04405 and WO 93/10125, British Patent Nos. 2,123,423 and 2,201,418, and U.S. Pat. Nos. 4,720,561 and 4,847,399.
The present invention relates to a recrystallization technique for the purification of an organometallic compound in which the organometallic compound is recrystallized, under cooling conditions, from a solvent containing the organometallic compound.
The present invention, while being described with particular reference to the purification of trimethylaluminum (TMAL), is also useful with other organometallic compounds, such as, but not limited to, the trialkyl compounds of aluminum, gallium or indium containing from one to about four carbon atoms in the alkyl group(s).
A common way in which TMAL, for example, is prepared is by the following reactions, conducted in batch (where Me signifies methyl and Et signifies ethyl):
2Al+3MeClxe2x86x92Me3Al2Cl3
2Me3Al2Cl3+2Et3Alxe2x86x923Me2AlCl+3Et2AlCl
3Me2AlCl+3Naxe2x86x922Me3Al+3NaCl+Al
The TMAL product from the above type of reaction will typically contain, for example, from about 50 to 20 about 200 ppm of silicon as an impurity due to the presence of such an impurity from the triethylaluminum and the methylaluminum-sesquichloride reagents that are employed in that process. The ultimate source of this impurity is from the aluminum powder that is used in the processes that are employed in making the triethylaluminum and methylaluminum-sesquichloride.
It has been found possible to purify the TMAL product of such impurity component, for example, by dissolving it in a suitable hydrocarbon solvent, such as iso-pentane, hexane, or heptane, and then cooling the resulting solution to a temperature of about 0xc2x0 C. or below to force the recrystallization of the TMAL out of the solution. The recrystallization procedure can be advantageously practiced at pressures ranging from low vacuum to moderate pressure (for example, from about 0 psia to about 45 psia). It is essential to the use of the present invention that the crystallization temperature of the organometallic compound (such as TMAL) needs to be higher than the solidification temperature of the selected solvent. Since the silicon impurities, for example, are apparently present in the TMAL as nonassociated and non-complexed silanes, these impurities will largely remain in the solvent, for example, after the TMAL has recrystallized from it. The liquor containing the impurities is then separated from the TMAL crystals either by decantation or filtration, and the recovered TMAL crystals can be washed with fresh solvent. The final step is, preferably, the melting and distillation of TMAL to remove the last traces of solvent from it.